I have pi with a RPi camera trained on the cat flap, when the python software detects movement [ by looking at the difference between pictures] it saves the picture. This way if one of the cats has not been seen for a while we at least know when it was last in and if it has visited while we were out. When we are away overnight I also run a second application which emails me a zip file with every time about 20 pictures have been captured. This way my wife gets a warm feeling the cats are around. I used the Pi cam rather than a PIR really just to minimise extra hardware. The problem with this setup is that you need to leave a light on to for it to work in the dark.

I have tried a PIR arrangement to trigger the a Pi noire camera which worked ok but if you rely on the Pi to power the IR LEDs you are a bit limited on power so I ended having to put the camera close to their food which worked. you can see the results here https://meanderingpi.wordpress.com/2015 ... t-monitor/

I do plan to develop this to include separate power for the IR LEDs but it is in the projects 'hopper' with a number of other things at the moment ...